In a typical wireless communication system, also referred to as cellular communication system or cellular radio system, user equipments, also known as mobile terminals and/or wireless terminals communicate via a Radio Access Network (RAN) to one Core Network (CN) or more CNs. The user equipments may be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability, e.g., mobile termination, and thus may be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called eNB, eNodeB, NodeB or B node and which in this document also is referred to as a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more CNs.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system (3G), which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. In the end of 2008 the first release, Release 8, of the 3GPP Long Term Evolution (LTE) standard was finalized and work on the Release 11 is currently going on.
Within the 3GPP specifications for LTE, the evolved radio access network is split into two parts: the Evolved UMTS Terrestrial Radio Access (E-UTRA), which describes the mobile part of LTE, and the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), which describes the base station part containing the Evolved Node B (eNB). Along with the LTE specifications, 3GPP is working on a complementary project called the System Architecture Evolution (SAE), which defines the split between LTE and a new Evolved Packet Core (EPC). This architecture is a flatter, packet-only core network that will help deliver the higher throughput, lower cost, and lower latency that is the goal of LTE. It is also designed to provide seamless interworking with existing 3GPP and non-3GPP access technologies.
Caching in wireless communication networks is a relatively known technology. Caching is based on that a large percentage of Internet traffic is repetitive and that eliminating repeating content all the way from its origin may offer an opportunity to reduce traffic and increase download speed. Therefore, the main principle is that copies of the content in e.g. the Internet are moved closer to the mobile users and stored in a cache, for example in different parts of the RAN, in the CN or just “above” the CN.
The main benefits that can be achieved with caching in wireless communication networks are:                Decrease the cost of transport in the wireless communication network. This is achieved “above the cache” as the cached information in principle is only transferred once in the transmission links above the cache.        Improved Quality of Experience for the mobile end-users. This is mainly achieved with lower delays as the cached information can be returned faster to the mobile users from the cache compared to if the information would be retrieved all the way from the original location.        Provide new services such as content hosting and storage/backup for operators. Mobile operators can sign agreements with the content providers that are based on that the mobile operator ensures that the content from a specific content provider is delivered in a better way to the mobile users in the mobile operator's network.        
Caching may also be used for the content distribution towards the mobile users. Instead of retrieving the downloadable content from the media server 101, the content can be retrieved from a network node located in a wireless communication system. The content from the media server may be pushed in to the network node in the wireless communication system and then the wireless user equipments will receive the content directly from the network node, instead of from a media server. It is also worth mentioning that caching can be used for almost any Internet content.
In an evolved architecture, a common network node for LTE and 3G is enabled by adding an alternative control interface between a network node and either a Mobility Management Entity (MME) or an eNodeB. The MME is a control node in the EPC core network for the LTE radio access network. The network node is common in the RAT-technologies and may be denoted “RNC-cache” as it may be placed for example in the RNC for 3G.
Caching as such may have long session times for playing a media stream such as e.g. WEB-TV with an hour long TV-program. A generation of content from the network node is in this document denoted “Cache-play-out”. During the period of an ongoing Cache-play-out, handover between RAT-technologies may occur.
Therefore, mobility support is required to support session-continuity for “Cache-play-out” from the network node. This applies for mobility scenarios such as e.g.:                Intra-RAT WCDMA Mobility        Intra-RAT LTE Mobility        Intra-RAT GSM Mobility        Inter-RAT Mobility between different RATs        
A User Equipment may have a set of ongoing cache play-out sessions, from now on denoted “cache sessions”. Each cache session is identified and described by a state-identifier denoted UE cache session. The content of the UE cache session depends of the type of network node and the type of content. The type of content may e.g. be video or audio. One example of UE cache session identifier may be the typical IP-flow description of five tuples, i.e. IP-source and IP-destination address, TCP or UDP source port, TCP or UDP destination port and Protocol (e.g. TCP or UDP), in combination with content specific identifiers.
A problem with Inter-RAT (IRAT) handovers is that the UE-cache session will be lost, since the source base station or source radio network controller and the target base station and target radio network controller can not recognize the UE-cache session if it is started in a common network node for another RAN than the base station or radio network controller itself.
The actions of an IRAT handover from LTE to WCDMA are briefly described below for the case when the same RNC-Network node is used on both RATs. It is worth noting that some actions and nodes are not described or are simplified but these do not impact the background discussion.
Initially the user equipment is active in LTE and the RNC-network node is performing play-out for at least one media stream. The UE session in the network node is known by identifiers specific for the LTE side and S1-interface. LTE uses its own temporary connection identifiers that are created by the MME “MME UE S1AP ID” and transferred in control signaling to the base stations. Within LTE network infrastructure, the S1 interface is the communications interface between an LTE base station and the EPC.
An IRAT handover is triggered from the source base station, i.e. an eNB in LTE, to a target RNC. The source base station selects the target UTRAN cell of WCDMA, for example based on measurements received from the user equipment. In this example the target RNC controlling the selected target UTRAN cell is the same RNC that comprises the current RNC-network node for the user equipment.
A handover request such as e.g. an S1 Application Protocol (S1AP) Handover Required message is sent from the source base station to the MME.
The MME forwards the received handover request to the target Serving General Packet Radio Service (GPRS) Support Node (SGSN). Please note that the handover request may e.g. be denoted “relocation request”, or “handover required” in some protocols.
The target SGSN forwards the handover request to the RNC controlling the selected target UTRAN cell and to the RNC-network node.
The problem is that the RNC-network node can not associate the received handover request or a relocation request with an active state in the network node, since it does not know that the handover request is related to the user equipment active in LTE. Therefore it can not continue the play-out from the network node after the handover is performed.